Houston, TX 6-hour Average Recurrence Interval for May 25-26, 2015

Over the past week, Texas has been inundated with heavy rain which has lead to devastating flooding in dozens of counties. This blog post will focus on the heavy rain event that occurred on May 25-26, 2015 centered in Houston, Texas.

The surface analysis map produced by the Weather Prediction Center shows the complexity of weather features that went into this event. A combination of surface features including a west-to-east oriented trough, a squall line, and two couplets of surface high and low pressure centers all can be seen in Oklahoma and Eastern Texas in this 21Z surface analysis.

As of 8AM May 29, 2015 all data including hourly and daily gauge, satellite, and radar data have been processed through MetStorm™. What is shown below is only preliminary as we are still making adjustments via our quality control process.

The maximum 6-hour precipitation, shown below, depicts a 6-hour maximum at each grid point. This being said, each grid point can be displaying a different 6-hour window of precipitation for the 24-hour storm duration. For the Houston, Texas storm, MetStorm™ calculated the maximum 6-hour precipitation to be 12.43″.

The Average Recurrence Interval, or ARI, uses this maximum 6-hour precipitation grid to calculate the frequency of occurrence for each point at a 6-hour frequency. MetStorm™ uses frequency grids from NOAA atlas volumns as well as other published studies and matches the precipitation amount to the frequency of occurrence. It should also be noted that the ARI calculation is not restricted to a 6-hour duration; it can be calculated for 1-hour, 2-hours, 3-hours, and so on.

The Houston, Texas 6-hour ARI revealed that the amount of rain that fell was climatologically significant, with an ARI of ~931 years. A common misconception about ARIs is that by saying this is a 931 year event, that it can only happen once in 931 years. This is untrue. From a climatological stand point it is simply the average number of years expected between events of this magnitude. An alternative way to understand ARIs is to think of them as a frequencies.

1/931=0.001 or 0.1%

So there is a 0.1% chance that in any given year there will be a rain event of this magnitude for this duration and location.